Diaphragms seal pressurizing fluid while transmitting a force across the seal. Traditional diaphragms are metal disks typically having a series of annular convolutions. The metal is chosen to be impervious to the process chemistry, and stainless steel is commonly used, as it withstands most process chemistries over temperature ranges from below freezing to several hundred degrees. The metal also forms a complete seal against the process fluid. Generally metal diaphrams are thick enough to be edge welded or otherwise sealed by conventional means to positioning and support structures. Welding assures secure positioning and complete sealing against edge leakage. The convolutions provide what can be typified as an accordial like flexibility perpendicular to the surface of the diaphragm. Ideally the convolutions provide a nearly zero spring rate over an appreciable range of diaphragm stroke. A planar metal diaphragm has an approximately linear displacement to force relation but has a useful spring rate over a limited range. Corrective signal processing methods can be used to reprocess and thereby extend the response of a metal diaphragm, but only at a significant cost.
Sensors now operate with sensitivities of less than one percent of full scale, and recent microfabrication techniques are making miniature devices possible. As a result the design limits of traditional diaphragms are being challenged. Metal diaphragms are too thick and too stiff to measure small pressure changes. Merely reducing the scale of a traditional metal diaphragm is not an effective solution. To be adequately flexible at small scale, a metal diaphragm must be extremely thin. A full scale metal diaphragm with a diameter of 9 cm and a thickness of 78 microns has a ratio of diameter to thickness of about 1000:1. A small diaphragm of a half centimeter in diameter with the same spring constant, would require a thickness of about 5 microns, which at 10 Angstroms per atomic layer, amounts to about 5000 atomic layers. The manufacture of such a thin layer with convolutions is extremely difficult to accurately achieve with uniformity. Further, corrosion, recrystallization, work hardening and similar proceses make the preservation of such a thin metal layer unlikely. Even if a small metal diaphragm could be made, sealing the diaphragm with a support structure is difficult. Welding or brazing do not appear to be possible as heat stress would likely puncture or warp the diaphragm. Adhesives are not effective over periods of time and are subject to the permeation problems characteristic of polymers.
Using a nonmetallic polymer material for a diaphragm may appear to be a reasonable option and in fact diaphragms have been made from such materials as leather, impregnated silk, fluorinated ethylene-propylene copolymer, neoprene and others. Polymerics have lower moduli of elasticity than metals, and are formable into thin sheets, offering low spring rate diaphragms. However, fluids and especially gases can pass through thin layers of flexible organic, or silicone based polymers by permeation. The resulting leakage may deleteriously affect performance. As with all diaphragms, chemical interaction between the material and the process can be a sgnificant problem. A particular material formulation may be impervious for one process but may not be generally useful. Heat is another problem for polymers. At low temperatures, crack fractures can occur, and at high temperatures melting and distortion can occur. Aging and crystallization may also affect polymers.
Accordingly, a need exists for a diaphragm to sense and transmit a spectrum of small pressure changes with minimal signal processing. Further, a material is needed that is both highly compliant over a long stroke, and fluid impermeable. Further a need exists for a diaphragm having a large displacement over a broad range of low pressures. Further, a need exists for such a material useful in micromechanical devices. Still further, a need exists for such a material to conveniently and completely seal along its edges. Further a need exists for a highly compliant diaphragm that is resistant over broad temperature extremes to chemical attack, and mechanical failure.